Shell-less cast-in-place concrete pile



May 28, 1968 R. v. JACKSON SHELL-LESS CAST-lN-PLACE CONCRETE PILE Filed July 14, 1965 /N VE A/TOR RICH/9 RD V. JACKSON AT1-o ,4M/Ks United States Patent O 3,385,070 SHELL-LESS CAST-lN-PLACE CONCRETE PILE Richard V. Jackson, Dallas, Tex., assignor to The Tecon Corporation, Dallas, Tex., a corporation of Delaware Filed July 14, 1965, Ser. No. 471,928 7 Claims. (Cl. 61-53.66)

ABSTRACT OF THE DISCLOSURE A method for installing cast-in-place concrete pile using a shell casing driven into the ground and then Vfilled with concrete. After the concrete has hardened, the shell is vibrated at a relatively high frequency and pulled out of the ground leaving a cast-in-place concrete piling of exact and uniform outer dimensions. The concrete piling may be made in the form of solid pilings or tubing and may be provided with pre-stressing tendons and means for posttensioning after the concrete has hardened and before the shell casing has been withdrawn from the hardened concrete piling.

It is common practice to install cast-in-place concrete piles in the ground by first installing, most commonly by driving methods, a hollow casing or shell which is subsequently filled with concrete. The form of the casing or shell varies in individual cases, some being of steel with sufficient wall thickness to withstand the driving stresses imposed during installation as Well as the external co1- lapsing forces imposed by the surrounding subsoil. Others take the form of thin Wall tubing, usually corrugated for additional strength in resisting external collapsing ground forces, but not designed to withstand the direct driving forces, the latter being transmitted through a mandrel or core temporarily inserted in the tubing during driving but withdrawn prior to placement of concrete. In these types of cast-in-place piling the casing or shell remains in the ground to act as a form for the poured in place concrete `and is not recoverable thereby adding to the cost of each pile installed.

To overcome the uneconomic practice of providing an unrecoverable casing or shell for each pile installed, methods have been devised to install shell-less concrete piles by initially installing a hollow casing of sufficient strength to withstand both the driving forces of installation and the external collapsing forces of the surrounding subsoil and by utilization of a detachable bottom end closure to exclude earth and water or other deleterious material during installation. Upon completion of installation of this casing the hollow interior of the same is lled with concrete but the withdrawing or removing of said casing is completed while the concrete is still in a uid state and prior to its having taken its initial set or hardening. The application of this type of pile is limited to certain types of subsoil conditions Where the external collapsing pressure imposed by the surounding subsoil is not so great as to cause displacement or necking down of the still fluid concrete column or where 4the surrounding subsoil is not of itself so soft or fluid as to permit expansion of the concrete column or actual mixing or contamination of the concerte by the surrounding subsoil.

A feature of the cast-in-place piles and in particular those in which the forming casing or shell remains in the ground and becomes part of the completed pile is the ability to visually inspect the hollow interior of the casing or shell for straightness, dryness and uniformity of crosssection prior to the placement of the fill concrete. Where the casing is withdrawn or recovered while the fill concrete is still iiuid, the final configuration and cross-section of the concrete column is dependent upon the characteristics ofthe surrounding subsoil and is not necessarily the same as the dimensions of the temporary casing.

3,385,070 Patented May 28, 1968 As will be more fully described hereinafter, this invention overcomes the economically objectionable practice of installing an unrecoverrable casing or shell and at the same time permits the installation of inspectable, shell-less concrete piles in any type of subsoil by virtue of the fact that the forming casing is removed or withdrawn only after the concrete has hardened to the point of being self-sustaining.

The invention employs the use of relatively high frequency vibratory units readily 'available from normal commercial sources that generate vibrations within the range of 15 to more than l0() cycles per second. These vibratory units are designed for use in the pile industry and are commonly used in both driving and extracting piles. In

the past these units have been used to install piles of the permanent shell type cast-inplace concrete variety described above and more often to install piles of the type known to those versed in the art as H-beam, Sheet Piles or Pipe Piles. These vibratory units have also been used as extracting tools employed in the removal of most known types of piling. IIn principle, the vibratory unit when firmly clamped to the pile transmits its vibrations through the pile and into the surrounding subsoil creating sufficient particle movement therein to disrupt frictional contact bet-Ween the adjacent surfaces of the pile exterior and the surrounding subsoil. In the driving sequence the vibrations must also create sufficient particle movement below the point of the pile to permit the pile, by virtue of its own weight combined with that of the vibratory unit, t0 penetrate the subsoil. In an extracting sequence, it is necessary only to break frictional contact between the adjacent surfaces of the pile exterior and the surrounding subsoil, thereby making the extracting sequence less dependent on the characteristics of the surrounding subsoil than is the driving sequence.

In the invention more specifically described hereinafter, the principle of breaking, by vibratory means, frictional contact between surfaces of unlike materials is applied t0 not only the adjacent surfaces of the casing exterior and the surrounding subsoil but also to the adjacent surfaces of the interior of the hollow casing and the concrete placed therein and allowed to harden after the casing has been installed in the ground.

The method employed in this invention of installing a shell-less cast-in-place concrete pile consists of installing by vibratory means, or the more conventional driving technique utilizing a steam, air or drop hammer, a hollow casing of suliicient strength to withstand the forces and stresses of installation as well as the strength to resist the external collapsing forces imposed by the surrounding subsoil, and having positioned but not fixedly attached at its lower end a suitable end'closure device of such configuration and strength as to exclude the infiltration of water, earth or other deleterious material from the hollow interior of the casing during and after installation of the casing. Many varieties of such end closure devices both as to conguration and material are well known to those familiar with or skilled in the art. Upon completion of the installation of the closed-end casing to the proper penetration and/or load bearing capacity, the hollow interior of the casing is filled with concrete and the concrete allowed to harden. After sufiicient time has elapsed for the concrete to harden, a vibratory unit is firmly attached to the upper end of the casing and, with the application of power, vibrations are transmitted to and through the casing. As vibrations commence the different modulii of elasticity of the two materials of the casing and the concrete contained therein creates relative movement between the two, resulting initially in breaking of whatever bond might have set up between the two materials. As vibration continues, particles of concrete immediately adjacent to the interior wall of the casing are fractured thereby disrupting frictional contact between the interior wall of the casing and the column of concrete positioned therein and permitting relative axial motion between the two. Simultaneously with the disruption of frictional contact between the interior surface of the casing and the concrete contained or positioned therein, there takes place a similar break down of frictional contact between the exterior surface of the casing and the adjacent surface of the surrounding subsoil or earth. Application of an upward axial force to the casing while vibration continues will result in the casing being withdrawn leaving the column of concrete with the end closure device at its lower end firmly in place in the ground. As the casing is withdrawn, while vibration thereof continues throughout the withdrawal operation, the vibration also serves to create sufficient particle movement in the surrounding subsoil or earth to move the surrounding subsoil into intimate contact with the surface of the concrete column lling the void left by the withdrawal of the casing, thereby establishing frictional contact between the concrete column and the surrounding subsoil, and resulting in what is known to those familiar with or skilled in the art as a friction pile, where subsoil characteristics are conducive thereto. Where subsoil characteristics are not conducive to a friction pile, an end-bearing pile also known to those skilled in the art and consisting of a full concrete column having the configuration and dimensions of the hollow interior of the casing uncontaminated by the surrounding subsoil results.

Application of the axial force to effect the withdrawal of the casing while the vibratory unit is in operation may be accomplished by but is not limited to the use of a lift crane, pile driver hoist or a hydraulic or mechanical jack, the means of application of force being merely supplementary to the invention.

As an aid in withdrawal of the casing, the invention includes the optional use of a bond inhibiting agent or material between the interior surface of the casing and the exterior surface of the cast-in-place concrete. This bond inhibiting agent may be, but is not limited to, any of the well known commercially avialable bond inhibiting agents applied to the interior surface of the casing at some time prior to the placement of the concrete within the confines of said casing. This bond inhibiting agent may take the form of the standard chemical mold release compounds, a plastic liner, a rubber liner, or coating and the like.

Although the weight or mass of the cast-in-place concrete column will normally be suicient to preclude its being raised or lifted as the casing is withdrawn, there may be some circumstances such as in the case of relatively short piles when it may prove advisable or advantageous to provide means of positively preventing any such lifting of the concrete column. The invention therefor includes such means which includes in the preferred method, but is not limited thereto, the introduction of fluid pressure, either gaseous or hydraulic, between the top of the concrete column and the underside of a removable cap or plug in the upper end of the casing. Since fluid pressure is exerted equally in all directions, this pressure will act not only to restrict the movement of the concrete column but will contribute to the axial movement vertically upward of the casing in relation to the concrete column, thereby reducing the required magnitude of the externally applied axial force previously described. Preventing movement of the concrete column during Withdrawal of the casing can also be accomplished by any means of applying a force or load vertically downward on the upper surface of the concrete column and acting axially concentric thereto.

The concrete piling may also employ the placement of plain or deformed reinforced concrete reinforcing steel bars embedded in the concrete column to improve the bending or tensile characteristics of the Concrete.

Additionally, formation of a prestressed cast-in-place concrete pile by either the pretensioning method or the post-tensioning method may be utilized. In the pretensioning method the lower end closing device and a temporary cap placed at the upper end of the casing contain the end anchorages for the pretensioned rods, wires or strands and the casing itself serves as a reactionary body to the pretensioning stresses prior to the hardening of the concrete and release of the pretensioning stresses thereto. In the post-tensioning method the lower end-closing device provides anchorage for the prestressing tendons and sleeves at the lower end while tensioning jacks react against the upper surface of the concrete column. Prestressing by either method is preferably completed prior to withdrawal of the casing.

The above features are objects of this invention and further objects will appear in the detailed description which follows and will otherwise be apparent to those skilled in the art.

For the purpose of illustration of this invention, there is shown in the accompanying drawing a preferred embodiment thereof. It is to be understood that these drawings are for the purpose of example only, however, and that the invention is not limited thereto.

In the drawings:

FIGURE 1 is a view in elevation showing an empty piling casing lowered onto a detachable end-closure device or point in preparation for vibrating or driving into the ground;

FIGURE 2 is a view in elevation showing the empty casing and point being installed in the ground. For purposes of illustration a conventional steam driven pile hammer is shown although a vibratory or other unit may be used;

FIGURE 3 is a cross sectional view in elevation showing the installed casing being filled with concrete;

FIGURE 4 is a cross sectional view in elevation showing the cast-in-place concrete being allowed to harden within the contines of the casing;

FIGURE 5 is a cross sectional view in elevation showing the casing partially withdrawn by means of attachment of a vibratory unit to the upper end of the casing and thel application of an axial upward acting force, the latter for purposes of illustration only being depicted as being supplied by a crane;

FIGURE 6 is a cross sectional view in elevation of a completed pile, the casing having been fully withdrawn and the surrounding subsoil moved into intimate contact with the concrete column completely filling the void left by the withdrawal of the casing;

FIGURE 7 is a cross sectional view in elevation of the upper end of the casing immediately prior to commencement of withdrawal showing a preferred method of sealing the upper end of the casing in preparation for the introduction of fluid pressure to the top of the concrete column in order to restrict movement thereof;

FIGURE 8 corresponds to FIGURE 7 and is a cross sectional view in elevation of the upper end of the casing showing the application of fluid pressure and indicating the force acting in opposite directions to produce relative motion between the casing and the column of concrete positioned therein;

FIGURE 9 is a cross sectional view in elevation showing reinforcing steel bars embedded in the concrete;

FIGURE 10 corresponds to FIGURE 9 but substitutes pretensioned prestressing tendons for the reinforcing steel bars shown in FIGURE 9; and

FIGURE 11 is a fragmentary cross sectional view in elevation showing construction of a hollow cast-in-place concrete pile.

Referring now to the drawings, a hollow casing 20 is shown in FIGURE 1 having an exterior surface 21 and an interior surface 22. For purposes of illustration only, the casing is shown as a hollow cylinder but is capable of being of any convenient or desirable cross-sectional configuration or shape positioned concentrically over and in contact with a removable or detachable end-closure device or point 23 in preparation for installation in the ground 24. Optionally the interior surface 22 of casing prior to or after installation in the ground, but prior to placement of concrete, may be coated or lined with a substance or material that would aid in the withdrawal of the casing 2Q from the hardened concrete as described hereinafter. This material or substance may take the form of but is not limited to one of the many commercially available bond-inhibitin g or form or mold release agents, or may be an actual liner of plastic, rubber, or some other substance that would effect the same purpose.

FIGURE 2 shows the casing 20 and point 23 being installed in the ground 24 for illustration purposes only by the application of driving forces of a conventional fluid actuated pile hammer 25, Forces are transmitted through a conventional driving head or follower 26 to the casing 26, it being understood that any applicable device, including but not limited to a vibratory unit that would effect satisfactory installation, might be employed in place of the pile hammer illustrated.

FIGURE 3 shows the installed hollow casing 20 being lled With concrete 27 in any acceptable manner and in the preferred and illustrated method through funnel 28, the uid concrete bearing directly upon the upper surface of point 23 and against the interior surface 22 of casing 20.

In FIGURE 4 the installed casing 20, in combination with point 23 and fill concrete 27, is allowed to remain undisturbed for a sufficient period of time to permit the fill concrete to harden.

In FIGURE 5 by attachment of a vibratory unit 29 through a clamp or other attachment device 31 to the upper end of the casing 26 and Iapplication of power to the vibratory unit 29 the frictional contact between the interior surface 22 of the casing 20 and the exterior surface 27' of the fill concrete 27 is reduced to approaching zero. Simultaneously, the frictional contact between the exterior surface 21 of casing 20 and the surface of the surrounding subsoil or ground is similarly reduced, and by application of an axial force acting upward supplied for illustration purposes only by attachment of a crane hook 33, the casing 2t) is separated from point 23 and withdrawn while the surrounding subsoil is moved by vibration through the void created by the withdrawal of casing 2li and into intimate contact with the exterior surface 21 of the hardened concrete column. Point y23 is retained in its installed position by the mass of concrete column 27 and forms a permanent part of the completed pile.

In FIGURE 6 casing 20 has been completely withdrawn and removed leaving the concrete column 27 bearing concentrically on point 23 and in frictional contact throughout the length and breadth of its exterior surface 27 with the surrounding subsoil.

FIGURE 7 shows a method of effecting a temporary seal in the upper end of casing 20 in order to permit the introduction of uid pressure to the top of the hardened concrete column should some means of preventing uplift or raising of said concrete column be required during the course of withdrawal of casing 20. The upper end of casing 2t) is provided with a plurality of shear blocks positioned radially against the inner surface 22 of said casing 20 and fixedly attached by welding or other means thereto in such a manner that the lower surface of all blocks are coplanar and provide a bearing surface for the upper removal plug plate of the sealing plug assembly. The size of each block and the space between adjacent blocks is sufficient to resist the bearing stress of the applied pressure while permitting the insertion and removal of the sealing plug assembly 32. Sealing plug assembly 32 consists of upper and lower plates 34 having positioned between them a disc or sheet 36 of resilient substance such as rubber, neoprene, or the like and being held together by a bolt and nut assembly 38. In the case of casing 20 being cylindrical in cross section the outside lll) diameter of plug plates 34 and disc 36 is only suiciently smaller than the inside diameter of casing 20 to permit installation of plug assembly 32 after the concrete column 27 has been placed. Upon positioning the plug assembly 32 below the shear blocks 36 and upon tightening the nut and bolt lassembly 38, the substance of the resilient disc 34 will be forced radially outward against the inner surface of the casing effecting a pressure tight seal against said inner surface. Subsequent loosening of the bolt and nut assembly will release such seal and permit removal of plug assembly 32. It is understood that many methods of effecting a temporary pressure tight seal may be devised or employe, and that shown is for purpose of illustration and is in no way limiting in the application of the spirit of this invention. A connection is also provided in the upper end of casing 20` below the plug assembly 32 and above the top surface of concrete column 27 to permit the attachment of a hose or pipe or other means of transporting the pressurized Huid to the cavity or charnber 42. This connection 40 might also be made through the plug plate assembly 32.

FIGURE 8 shows a r'luid 44 within the cavity pressurized by some exterior means such as a fluid pump (not shown) sufficiently to effect relative axial movement between the casing 20 and the concrete column 27. The force of the pressurized yfluid is exerted equally in all directions being upward against the lower surface of sealing plug assembly 32 and downward against the top surface of the concrete column as well as laterally against the inner surface of the casing. Casing 20 is sufficiently strong and rigid to resist the bursting effect thereon of the pressurized fluid and the net result is the effective restraint of movement of the concrete column which is supported against downward movement by the resistance of the ground beneath point 23 and the withdrawal of the casing 20 from its position of encasement of the concrete column 27.

In FIGURE 9 reinforcing steel bars 46 are shown embedded in the concrete column 27 during the placement of said concrete to provide additional strength in bending or tension should it be required. For purposes of illustration only, bars 46 are shown extending the full length of the concrete column, it being understood that these bars may be as short or as long as required and may be a single bar or a plurality of bars positioned within the concrete column, as required, and, if necessary, extending above the top surface of the column to provide anchorage for a subsequently placed cap or for any other purpose.

In FIGURE l0 a bottom end anchorage 48 is provided to point 23 to accept and hold the lower end of a pretensioned prestressing tendon 50 or a plurality of the same. A removable or temporary bridge 52, provided with a similar top end anchorage device 54, is positioned and bears concentrically on the upper end of casing 20. The tendons 50 are pretensioned in a conventional manner common to the art of prestressing concrete and the necessary reactionary forces are supplied by the casing 20 acting as a column in compression. Upon completion of the pretensioning operation, and locking of the pretensioning stresses in the tendons, fill concrete 27 is poured to fully encompass the prestressing tendons 50 and allowed to harden sufficiently at which time the tendons are released from their end anchorage 54 in bridge 52 and the prestress is transferred to the ll concrete 27 in the manner nonmal to the art of prestressing concrete. Release of the tendons 50 from end anchorage 54 permits the removal of the temporary bridge 52 and withdrawal of the casing may proceed by means of the attachment of the vibratory unit 29, as previously described under. FIG- URE 5. It will be obvious to those familiar with the art that only minor modifications in procedure would be required to adapt the preceding described process of pretensioning the prestressing tendons to the commonly known process of post-tensioning.

It will be noted that the configuration and dimensions of the hollow casing used in the process employed in this invention can be varied as desired. Thus, it may have various cross sectional configurations, such as circular, fiuted, square, a polygon, or the like, and can `be designed to withstand any of the stresses which may be imposed upon it whether these stresses occur during or after installation or during the withdrawal operation. The casing may also have either a constant cross section throughout its length, or it may taper slightly, being slightly smaller in the interior cross-section at its upper end to further aid in the reduction of frictional contact during the withdrawal operation. Likewise, the detachable end-closure device or point may vary in design, configuration and dimensions, while serving its primary function of being detachable and effectively sealing the lower end of the casing against infiltration of outside material, water, or any deleterious matter into the casing interior during and after installation in the ground. It may be of precast concrete, fabricated steel, cast steel or cast iron, or other conventional materials -used in this construction.

It will be noted that an improved method of installation of an inspectable shell-less cast-in-place pile has been disclosed. Upon completion of installation of the hollow casing and point 23 in the ground, and prior to the placement of concrete therein, visual or other inspection of the interior of casing 20 may be made for the purpose of determining straightness, dryness and uniformity of cross section with the assurance that the concrete subsequently placed within the inspected cavity and allowed to harden will retain the same cross-sectional and longi tudinal configurations and dimensions of the hollow interior of casing 20 regardless of the characteristics of the surrounding subsoil.

The invention further provides a more economical end product pile in that the necessity of using a costly unrecoverable shell or casing to assure dimensional uniformity of the cast-in-place concrete column is eliminated.

The invention also discloses a practical and economical method of constructing cast-in-place concrete piling for marine construction where the upper portion of the pile is surrounded by water only or other fluid material and its lower portion extends to adequate bearing subsoil or even bedrock. The removal of the casing only after the fill concrete has hardened to the point of being self-supporing eliminates the necessity of leaving a forming shell or casing of steel or other material in place and susceptible to corrosive action and the subsequent unsightliness well known in this type of construction.

A modification of the invention includes the construction of a hollow cast-in-place concrete pile shown in FIG- URE 1. This is accomplished by the use of an inner form 60 self-supporting throughout its entire length placed within the casing 20` and concentric therewith, its exterior surface being sufficiently separated `from the interior surface of the casing to form an annular void therebetween into which fluid concrete 27 is placed. After said concrete has hardened both the casing and the inner form are withdrawn or removed in the manner disclosed in this invention resulting in a hollow concrete shell pile. The void or hollow interior may -be subsequently filled with concrete, gravel, stones, sand or other material, or may be simply plugged at the top to provide a `full bearing area.

A further modification of the invention includes the construction or manufacture of precast concrete piling either solid or hollow by adapting the invention to the removal of forms or molds used in the construction of said piling. Precast piling of this type is usually constructed above ground surface for later installation in the ground by driving or other conventional installation techniques.

It will be noted that various embodiments of the invention have been illustrated. It will be obvious that such embodiments are subject to modifications, which will be readily apparent to those skilled in the art and will fall within the spirit of the invention. Such modifications are within the scope of the invention as defined by the fotlowing claims.

What is claimed is:

1. A method of constructing cast-in-place concrete piling in the ground which comprises pouring or placing concrete in a hollow casing having a uniform cross-section installed in a ground formation, allowing the concrete to harden, and withdrawing the casing from the so formed concrete piling and the ground formation by vibrating the casing to disrupt bonding and frictional forces simultaneously between both the casing and the concrete piling and the casing and the surrounding ground formation while exerting an axial force on said casing to withdraw it from the concrete piling and the ground formation, said axial force being exerted by the application of tiuid pressure between the top of the concrete piling and the underside of a sealing plug placed in the upper end of the casing, such fluid pressure being contained laterally by the casing walls exerting a downward force upon the concrete piling and an upward force upon the casing thereby effecting its withdrawal.

2. A method of constructing cast-in-place concrete piling in the ground which comprises filling a hollow casing having a uniform cross-section installed in a ground formation with fluid concrete, allowing the concrete to harden, and removing the casing from the so formed concrete pile by vibrating said casing to disrupt bonding and frictional forces simultaneously between both the casing and the concrete piling and the casing and the surrounding ground formation while exerting a downward force on the top of the concrete piling to prevent its upward movement and simultaneously exerting an upward force on the casing to effect its withdrawal from the concrete column.

3. A method of constructing cast-in-place concrete piling in the ground which comprises filling a hollow casing having a uniform cross-section installed in a ground formation with fluid concrete, allowing the concrete to harden, and removing the casing from the so formed concrete pile by vibrating said casing to disrupt bonding and frictional forces simultaneously between both the casing and the concrete piling and the casing and the surrounding ground formation while exerting axial forces simultaneously downward upon the top of the concrete piling and upward upon the casing to effect withdrawal of the casing, such forces being exerted by the introduction of fluid pressure between the top of the concrete piling and the underside of a sealing plug installed in the upper end of the casing, such pressure being contained laterally by the casing walls exerting a downward force on the top of the concrete piling and an upward force on the casing such upward force acting simultaneously to complement and thereby reduce the required magnitude of any other external force applied upwardly to the casing to effect its withdrawal from the piling.

4. A method of constructing a hollow cast-in-place concrete shell pile in the ground which comprises installing a hollow casing having a uniform cross-section having a detachable end-closure device at its lower end in a ground formation, positioning concentrically within said casing an inner forming device smaller in cross-sectional dimension than said casing to form a cavity between the casing and the inner form, filling said cavity with fiuid concrete and after allowing said concrete to harden withdrawing the casing by vibrating the same to disrupt bonding and frictional forces simultaneously between the casing and the concrete shell and between the casing and the ground formation while exerting an upwardly directed axial force upon said casing to effect its withdrawal from its position of encasement of the concrete shell and from the ground, and in turn withdrawing the inner form by similarly vibrating the same while exerting an upward axial force to effect its withdrawal from its position within the concrete shell.

S. A method of constructing a hollow cast-in-place concrete shell pile in the ground which comprises installing a hollow casing having a uniform cross-section having a detachable end-closure device at its lower end in a ground formation, positioning concentrically within said casing an inner formin-g device smaller in cross-sectional dimension than said casing to create a cavity between the casing and the inner form, filling said cavity with fluid concrete and after allo-wing said concrete to harden withdrawing the inner form by vibrating to disrupt bonding and frictional forces between the inner form and the concrete and exerting an axial force upward upon said inner form to effect its withdrawal from within the concrete shell and in turn withdrawing the casing by similarly vibrating the same to disrupt .bonding and frictional forces both between the casing and the concrete shell and between the casing and the ground formation while exerting an upward force upon said casing to effect its withdrawal from its position of encascment of the concrete shell and from the ground formation.

6. A method of constructing a cast-in-place prestressed concrete pile in the ground which comprises installing a hollow casing having a uniform cross-'section in the ground, said casing having positioned at its lower end a detachable end closure device tted with a suitable end anchorage for a concrete prestressing tendon or a plurality thereof and said casing having positioned at its upper end a detachable yoke or bridge similarly fitted with a device or a plurality thereof capable of accepting prestressing tendons, and after application of the tensioning forces to the tendons, gripping said tendon to lock the applied stresses therein, the necessary reactionary forces being supplied by compression of the casing, and after so pretensioning the tendons lling the hollow interior of said casing with uid concrete, allowing said concrete to harden and subsequently releasing the end anchorage at the upper end of the tendons to transfer the pretensioning stresses to said concrete, thereafter proceeding with the removal of the upper bridge or yoke and withdrawal of said casing by vibrating the same to disrupt bonding and frictional forces both between the casing and the concrete and between the casing and the ground formation While exerting an upward axial force upon said casing to effect the withdrawal of the casing from its position of encasement of the concrete pile and from the ground.

7. A method of constructing a cast-in-place prestressed concrete pile in the ground which comprises installing a hollow casing having a uniform cross-section in a ground formation, said casing having positioned at its lower end an end-closure device fitted with a suitable device or a plurality thereof to accept and lock in place the tendons and tendon-sleeves commonly used in post-tensioning prestressed concrete, said casing having positioned at its upper end a device to position such tendon and sleeve assembly or assemblies in proper relation to the casing interior prior to and during the lling of said casing interior with Huid concrete and said concrete having been placed and allowed to harden, thereafter applying tensile forces to the tendons, using the concrete column as a reaction, locking said tensile stresses in said tendons and grouting the void between the tendons and their surrounding sleeves and after hardening of said grout releasing the upper end locking devices thereby transferring the post-tensioning stresses to the concrete pile, thereafter proceeding with the withdrawal of the casing by vibrating to disrupt bonding and frictional forces both between the casing and the concrete and between the casing and the ground formation while exerting an upward axial force upon said casing to effect the withdrawal of the casing from its position of encasement of the concrete pile and from the ground.

References Cited UNITED STATES PATENTS 1,454,434 5/1923 Gardner 61--53.66 1,629,622 5/1927 Hiley 61-53.64 3,187,513 6/1965 Guild 61--5352 3,192,292 6/1965 Banks 264-32 3,198,857 8/1965 Childers et al. 61-53.66 X 3,239,005 3/ 1966 Bodine 166--23 FOREIGN PATENTS 428,119 6/1911 France.

676,201 7/ 1952 Great Britain.

715,719 9/1954 Great Britain.

75,343 7/ 1954 Netherlands.

JACOB SHAPIRO, Primary Examiner. 

